Method of depositing amorphous silicon based films having controlled conductivity

ABSTRACT

Deposition methods for preparing amorphous silicon based films with controlled resistivity and low stress are described. Such films can be used as the interlayer in FED manufacturing. They can also be used in other electronic devices which require films with controlled resistivity in the range between those of an insulator and of a conductor. The deposition methods described in the present invention employ the method of chemical vapor deposition or plasma-enhanced chemical vapor deposition; other film deposition techniques, such as physical vapor deposition, also may be used. In one embodiment, an amorphous silicon-based film is formed by introducing into a deposition chamber a silicon-based volatile, a conductivity-increasing volatile including one or more components for increasing the conductivity of the amorphous silicon-based film, and a conductivity-decreasing volatile including one or more components for decreasing the conductivity of the amorphous silicon-based film.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of U.S. application Ser. No. 08/500,728,filed Jul. 11, 1995, now U.S. Pat. No. 5,902,650.

This application is a continuation-in-part of U.S. Pat. No. 5,902,650,filed Jul. 11, 1995, and entitled “Method of Depositing AmorphousSilicon Based Films Having, Controlled Conductivity,” which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention generally relates to a method of depositing on asubstrate an amorphous silicon based film that has controlled electricalconductivity and more particularly, relates to a method of depositing anamorphous silicon based film that has controlled conductivity in betweenthat of an intrinsic amorphous silicon and an n⁺ doped amorphoussilicon. The film may be deposited onto a substrate by a chemical vapordeposition process.

In recent years, flat panel display devices have been developed for usein many electronic applications including notebook computers. One suchdevice, an active matrix liquid crystal display, has been usedfrequently. However, the liquid crystal display device has inherentlimitations that render it unsuitable for many applications. Forinstance, liquid crystal displays have fabrication limitations such as aslow deposition process of amorphous silicon on glass, highmanufacturing complexity and low yield. The displays require apower-hungry fluorescent backlight while most of the light is wasted. Aliquid crystal display image is also difficult to see in bright sunlightor at extreme viewing angles which present a major concern in manyapplications.

A more recently developed device of a field emission display (FED)overcomes some of these limitations and provides significant benefitsover the liquid crystal display devices. For instance, the FEDs havehigher contrast ratio, larger viewing angle, higher maximum brightness,lower power consumption and a wider operating temperature range whencompared to a typical thin film transistor liquid crystal displaydevice.

Unlike the liquid crystal displays, field emission displays (FEDs)produce their own light using colored phosphors. The FEDs do not requirecomplicated, power-consuming backlights and filters and almost all thelight generated by an FED is visible to the user. The FEDs do notrequire large arrays of thin-film transistors. A major source of yieldproblems for active matrix liquid crystal displays is thereforeeliminated.

In a FED, electrons are emitted from a cathode and impinge on phosphorson the back of a transparent face plate to produce an image. It is knownthat such a cathodoluminescent process is one of the most efficient waysfor generating light. Unlike a conventional CRT, each pixel in an FEDhas its own electron source, typically an array of emitting microtips.The voltage difference between the cathode and the gate extractselectrons from the cathode and accelerates them towards the phosphors.The emission current and thus the display brightness, is stronglydependent on the work function of the emitting material. The cleanlinessand uniformity of the emitter source material are therefore essential.

Most FEDs are evacuated to low pressures, i.e., 10⁻⁷ torr, to provide along mean free path for emitted electrons and to prevent contaminationand deterioration of the tips. Display resolution is improved by using afocus grid to collimate the electrons drawn from the microtips.

The first field emission cathodes developed for a display device used ametal microtip emitter of molybdenum. In such a device, a silicon waferis first oxidized to produce a thick SiO₂ layer and then a metallic gatelayer is deposited on top of the oxide. The gate layer is then patternedto form gate holes. Etching the SiO₂ underneath the holes undercuts thegate and creates a well. Molybdenum is deposited at normal incidenceand, at the same time, a sacrificial material such as Ni is depositedfrom a source placed at the side of the device such that cones withsharp points grow inside the cavities. Emitter cones are left when thesacrificial layer is removed.

In another FED device, silicon microtip emitters are produced bythermally oxidizing a silicon substrate, patterning the silicon oxide toexpose the underlying silicon substrate, and selectively etching theexposed silicon to form silicon tips. Further oxidation and etchingprotects the silicon and sharpens the points of the silicon tips.

In an alternative design, the microtips are added onto a substrate ofdesired materials such as glass, which is an ideal substrate materialfor large area flat panel display. The microtips can be made ofconducting materials such as metals or doped semiconductors. In such aFED device, an interlayer with controlled conductivity between thecathode and the microtips is highly desirable. Proper engineering of theresistivity of the interlayer enables the device to operate in a stableand controllable fashion. The resistivity of the interlayer is in theorder between an insulator and a conductor, while the actual desiredvalue depends on the specifics of the device design.

Chemical vapor deposition (CVD) or plasma-enhanced chemical vapordeposition (PECVD) are processes widely used in the manufacture ofsemiconductor devices for depositing layers of materials on varioussubstrates. In a conventional PECVD process, a substrate is placed in avacuum deposition chamber equipped with a pair of parallel plateelectrodes. The substrate is generally mounted on a susceptor which isalso the lower electrode. A reactant gas flows into the depositionchamber through a gas inlet manifold which also serves as the upperelectrode. A radio frequency (RF) voltage is applied between the twoelectrodes which generates an RF power sufficient to cause a plasma tobe formed in the reactant gas. The plasma causes the reactant gas todecompose and deposit a layer of the desired material on the surface ofthe substrate body. Additional layers of other electronic materials canbe deposited on the first layer by flowing into the deposition chamber areactant gas containing the material of the additional layer to bedeposited. Each reactant gas is subjected to a plasma which results inthe deposition of a layer of the desired material.

In the fabrication of a field emission display device, it is desirableto deposit an amorphous silicon based film that has electricalconductivity in an intermediate range between that of intrinsicamorphous silicon and n⁺ doped amorphous silicon. The conductivity ofthe n⁺ doped amorphous silicon is controlled by adjusting the amount ofphosphorus atoms contained in the film. Even though it is possible, inprinciple, to produce an intermediate conductivity film by adding verysmall amounts of phosphorus atoms, it is a very difficult task, i.e.requires specially premixed PH₃/H₂ to precisely control the amounts ofthe phosphorus atoms.

Since field emitting display devices use very thick layers, it becomesnecessary to deposit low stress films to prevent warping of the glassand peeling of the films. The standard process for depositing amorphoussilicon produces films that are highly compressive, especially whendeposited at high deposition rates.

SUMMARY OF THE INVENTION

The present invention provides a deposition method for preparingamorphous silicon based films with controlled resistivity and lowstress. Such films can be used as the interlayer in the FEDmanufacturing. They can also be used in other electronic devices whichrequire films with controlled resistivity in the range between those ofan insulator and of a conductor. The deposition method described in thepresent invention employs the method of chemical vapor deposition orplasma-enhanced chemical vapor deposition; other film depositiontechniques, such as physical vapor deposition, also may be used.

In one aspect, the invention features a method of forming an amorphoussilicon-based film on a substrate located inside a deposition chamber,comprising: introducing a silicon-based volatile into the depositionchamber; introducing into the deposition chamber aconductivity-increasing volatile comprising one or more components forincreasing the conductivity of the amorphous silicon-based film; andintroducing into the deposition chamber a conductivity-decreasingvolatile comprising one or more components for decreasing theconductivity of the amorphous silicon-based film.

In another aspect, the invention features a method of forming anamorphous silicon-based film on a substrate located inside a depositionchamber, comprising: introducing a silicon-based volatile into thedeposition chamber; introducing phosphine into the deposition chamber;and introducing a nitrogen-containing volatile into the depositionchamber.

In yet another aspect, the invention features a method of forming anamorphous silicon-based film on a substrate located inside a depositionchamber, comprising: introducing a silicon-based volatile into thedeposition chamber; introducing phosphine into the deposition chamber;and introducing a carbon-containing volatile into the depositionchamber.

Embodiments may include one or more of the following features.

The conductivity-increasing volatile and the conductivity-decreasingvolatile may be introduced into the deposition at respective relativeflow rates selected to achieve a desired film resistivity. The relativeflow rates may be selected to achieve a film resistivity of about10³-10⁷ ohm-cm. The conductivity-increasing volatile may consist ofphosphine and the conductivity-decreasing volatile may consist ofammonia, the phosphine and the ammonia being introduced into thedeposition chamber at a flow rate ratio in a range of about 1:1000 toabout 1:10 (phosphine:ammonia). Alternatively, theconductivity-increasing volatile may consist of phosphine and theconductivity-decreasing volatile may consist of methane, the phosphineand the methane being introduced into the deposition chamber at a flowrate ratio in a range of about 1:100 to about 1:1 (phosphine:methane).

The conductivity-increasing volatile may comprise a dopant. The dopantmay comprise an n-type dopant (e.g., phosphorous) or a p-type dopant(e.g., boron).

The amorphous silicon-based film may be characterized by a band gap, andthe conductivity-decreasing volatile preferably comprises a band gapincreasing component that increases the band gap of the amorphoussilicon-based film relative to a film formed under similar conditionsbut without the band gap increasing component. Theconductivity-decreasing volatile may comprises nitrogen, ammonia, N₂,N₂O, carbon (e.g., methane).

In one embodiment, the silicon-based film consists of silane, theconductivity-increasing volatile consists of phosphine, and theconductivity-decreasing volatile consists of ammonia. In anotherembodiment, the silicon-based film consists of silane, theconductivity-increasing volatile consists of phosphine, and theconductivity-decreasing volatile consists of methane. In yet anotherembodiment, the silicon-based film consists of silane, theconductivity-increasing volatile consists of phosphine, the firstconductivity-decreasing volatile consists of ammonia, and the secondconductivity-decreasing volatile consists of methane.

A second conductivity-decreasing volatile may be introduced into thedeposition chamber.

In a preferred embodiment, amorphous silicon based films of preciselycontrolled electrical conductivity and low stress are produced byflowing a reactant gas mixture into a plasma-enhanced chemical vapordeposition chamber. The reactant gas mixture comprises silane, ammoniaand phosphine carried by a hydrogen gas. Changing the phosphorus contentby controlling the phosphine partial pressure, the n-type electricalconductivity of the amorphous silicon based film can be changed, i.e.increasing the phosphorus content increases the electrical conductivity.Changing the nitrogen content of the reactant gas by controlling theammonia partial pressure, the resistivity can be changed, i.e.increasing the nitrogen content increases the resistivity of theamorphous silicon based film. An ideal range of resistivity for thefield emission display devices is between about 10³ and about 10⁷ohm-cm. The novel method described in this invention enables one tocontrol the resistivity of an amorphous silicon based film within thedesirable range of 10³ to 10⁷ ohm-cm. The films produced by the novelmethod have low tensile stress such that warping or peeling of filmsfrom substrates are avoided.

The present invention is also directed to a field emission displaydevice fabricated by a plasma-enhanced chemical vapor depositiontechnique in which a reactant gas mixture comprising silane, hydrogen,phosphine (carried in hydrogen) and ammonia is used to produce anamorphous silicon based film having controlled electrical conductivity.By adjusting the flow rate of each component gas, an amorphous siliconbased film having a precisely controlled electrical conductivity and lowstress for forming a field emission device can be obtained.

Among the advantages of the invention are the following.

The invention provides a method of depositing amorphous silicon basedfilms that have controlled conductivity and low stress in a chemicalvapor deposition process or a plasma-enhanced chemical vapor depositionprocess by incorporating simple process control steps. The inventionalso provides a method of depositing amorphous silicon based films thathave controlled conductivity and low stress in a chemical vapordeposition process or a plasma-enhanced chemical vapor depositionprocess by using a reactant gas mixture containing PH₃ and NH₃. Thepresent further provides a method of depositing amorphous silicon basedfilms that have controlled conductivity and low stress in a chemicalvapor deposition process or a plasma enhanced chemical vapor depositionprocess by controlling the flow rates of the reactant gases in thereaction chamber.

Other features and advantages will become apparent from the followingdescription, including the drawings and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a plasma-enhanced chemical vapordeposition chamber in which the method in accordance with the presentinvention can be carried out.

FIG. 2 is an enlarged cross-sectional view of a typical field emissiondisplay device.

FIG. 3 is a graph illustrating the dependence of resistivity on the flowrate ratio between PH₃ and NH₃.

FIG. 4 is a graph illustrating the dependence of stress on the flow rateratio between PH₃ and NH₃.

FIG. 5 is a graph of the resistivity of amorphous silicon-based filmsplotted against the flow rate ratio phosphine:methane.

DETAILED DESCRIPTION

The present invention discloses an improved method of depositingamorphous silicon based films having controlled electrical conductivityand low stress on a substrate for an electronic device, such as a fieldemission display device by a plasma enhanced chemical vapor depositiontechnique.

Referring initially to FIG. 1, there is shown a schematic sectional viewof a plasma-enhanced chemical vapor deposition apparatus 10 in which themethod in accordance with the present invention can be carried out.Turner et al. disclose such an apparatus in U.S. Pat. No. 5,512,320. Adeposition chamber 12 includes an opening through a top wall 14 and afirst electrode or a gas inlet manifold 16 within the opening.Alternatively, the top wall 14 can be solid with the electrode 16 beingadjacent to the inner surface thereof. Within chamber 12 is a susceptor18 in the form of a plate which extends parallel to the first electrode16. The susceptor 18 is typically of aluminum and is coated with a layerof aluminum oxide. The susceptor 18 is connected to ground so that itserves as a second electrode so as to connect the RF source 36 acrossthe two electrodes 16 and 18.

The susceptor 18 is mounted on the end of a shaft 20 which extendsvertically through a bottom wall 22 of the deposition chamber 12. Theshaft 20 is movable vertically so as to permit the movement of thesusceptor 18 vertically toward and away from the first electrode 16. Alift-off plate 24 extends horizontally between the susceptor 18 and thebottom wall 22 of the deposition chamber 12 substantially parallel tothe susceptor 18 and is vertically movable. Lift-off pins 26 projectvertically upwardly from the lift-off plate 24. The lift-off pins 26 arepositioned to be able to extend through lift holes 28 in the susceptor18, and are of a length slightly longer than the thickness of thesusceptor 18. While there are only two lift-off pins 26 shown in thefigure, there may be more lift-off pins 26 spaced around the lift-offplate 24.

A gas outlet 30 extends through a side wall 32 of the deposition chamber12 and is connected to means (not shown) for evacuating the depositionchamber 12. A gas inlet pipe 42 extends into the first electrode or thegas inlet manifold 16 of the deposition chamber 12, and is connectedthrough a gas switching network (not shown) to sources (not shown) ofvarious gases. The first electrode 16 is connected to an RF power source36. A transfer paddle (not shown) is typically provided to carrysubstrates through a load-lock door (not shown) into the depositionchamber 12 and onto the susceptor 18, and also to remove the coatedsubstrate from the deposition chamber 12.

In the operation of the deposition apparatus 10, a substrate 38 is firstloaded into the deposition chamber 12 and is placed on the susceptor 18by the transfer paddle (not shown). The substrate 38 is of a size toextend over the lift holes 28 in the susceptor 18. A commonly used sizefor a flat panel display device substrate is approximately 360 mm by 465mm. The susceptor 18 is positioned above the lift-off pins 26 by movingshaft 20 upwards such that the lift-off pins 26 do not extend throughthe holes 28, and the susceptor 18 and substrate 38 are relatively closeto the first electrode 16. The electrode spacing or the distance betweenthe substrate surface and the discharge surface of the gas inletmanifold 16 is between about 12 to about 50 mm. A more preferredelectrode spacing is between about 20 to about 36 mm.

Before the deposition process of the invention, the substrate 38, whichmay be a large sheet of a transparent material, is processed accordingto well known techniques. After the initial processing in the preferredembodiment, a top most layer containing a patterned metal is deposited.

At the start of the deposition process of the invention, the depositionchamber 12 is first evacuated through the gas outlet 30. The patternedsubstrate 38 is then positioned on the susceptor 18. The substrate 38 iskept at a temperature between about 200° C. and about 400° C. duringdeposition of the present invention amorphous silicon film. A preferredtemperature range for the substrate is between about 300° C. and about350° C. during deposition. A pressure is maintained in the reactionchamber during deposition at between about 0.5 torr and about 5 torr, apreferred pressure range is between about 1.5 torr and about 2.5 torr. Adetailed description of processing with this apparatus is contained inU.S. Pat. No. 5,399,387 Law et al. assigned to the common assignee whichis hereby incorporated by reference in its entirety.

FIG. 2 shows an enlarged cross-sectional view of a typical fieldemission display device 50. The device 50 is formed by depositing aresistive layer 52 of amorphous silicon based film on a glass substrate54. An insulator layer 56 and a metallic gate layer 58 are thensequentially formed and etched in such a way as to form metallicmicrotips 60. A cathode structure 62 is covered by the resistive layer52. Thus, a resistive but somewhat conductive amorphous silicon layer 52underlies a highly insulating layer 56, such as of SiO₂. It is importantto be able to control the resistivity of the amorphous silicon layer 52so that it is not overly resistive but it will act as a limitingresistor to prevent excessive current flow if one of the microtips 60shorts to the metal layer 58.

It should be noted that while a field emission display device is shownhere to demonstrate the present invention method, the method is by nomeans limited to the fabrication of FEDs. The present invention methodcan be used in the fabrication of any electronic devices that require adeposition of a layer having controlled resistivity.

A series of tests were conducted on test samples prepared by the presentinvention method to determine the effects of the reactant flow ratesupon the conductivity and the stress of the films produced. Theirresults are summarized in Tables 1 and 2.

EXAMPLE 1

Example 1 illustrates a deposition process for an intrinsic amorphoussilicon film that does not contain doping gases in the reactant gasmixture. The film is formed as a byproduct of a reaction of silane andhydrogen in a plasma on a heated substrate. The flow rate of silane iscontrolled at 1,000 sccm, the flow rate of hydrogen is controlled at1,000 sccm, the plasma power (i.e. RF power supplied to electrode 16)used is 300 W, the pressure of the chamber is kept at 2.0 torr, theelectrode spacing (i.e., the spacing between electrode 16 and susceptor18) is kept at 962 mils (2.44 cm), the susceptor, which heats thesubstrate by contact heating, is maintained at a temperature of 410° C.,and the deposition rate obtained is 168 nm/min. After the completion ofthe deposition process, the films obtained were tested to produce thefollowing physical properties, a compressive stress of 8.0×10⁸ dyne/cm²and a resistivity of 2.0×10⁹ ohm-cm.

EXAMPLE 2

Example 2 illustrates a deposition process for p-doped amorphous siliconwherein the film was deposited with a flow of PH₃ gas but with no NH₃gas. The flow rate of silane is 1,000 sccm, the flow rate of PH₃ is 0.5sccm, the flow rate of hydrogen is 1000 sccm, the RF power used is 300W, the pressure of the chamber is 2.0 Torr, the susceptor spacing is 962mils (2.44 cm), the susceptor temperature is 410° C., and the depositionrate achieved is 156 nm/min. The phosphine is a 0.5% concentration in ahydrogen carrier that is otherwise accounted for in the cited flowrates. The amorphous silicon film obtained has a compressive stress of1.7×10⁹ dyne/cm² and a resistivity of 1.7×10² ohm-cm.

EXAMPLE 3

In this example, only ammonia gas and not PH₃ gas is used in thereactant gas mixture. The flow rate of silane gas used is 1,000 sccm,the flow rate of NH₃ is 500 sccm, the flow rate of hydrogen gas is 1,000sccm, the RF power used is 300 W, the pressure of the chamber is kept at2.0 Torr, the electrode spacing is 962 mils (2.44 cm), the temperatureof the susceptor is 410° C., and a deposition rate of 135 nm/min wasobtained. The physical properties obtained for the film are quitedifferent than that obtained in Example 2. The stress of the film is ina tensile mode of 7.4×10⁹ dyne/cm². The resistivity of the film measuredhas a high value of 2.2×10¹⁰ ohm-cm.

EXAMPLE 4

In this example, a doping gas of PH₃ and a nitrogen-containing gas of NH₃ are used in the reactant gas mixture. The flow rate ratio of PH₃ toNH₃ is 1.25×10⁻² to 1. The flow rate of silane is 1,000 sccm, the flowrate of PH₃ is 2.5 sccm, the flow rate of NH₃ is 200 sccm, the flow rateof H₂ is 1,000 sccm. The RF power used is 600 W, the pressure of thechamber is 2.0 Torr, the spacing of the electrodes is 962 mils (2.44cm), the susceptor temperature is 400° C., and the deposition rateobtained is 197 nm/min. An amorphous silicon film having a desirableresistivity is obtained. The physical properties of the film aremeasured at a tensile stress of 4.0×10⁸ dyne/cm² and a resistivity of1.6×10⁵ ohm-cm. It is noted that the resistivity value is about halfwaybetween the two extreme values shown in Example 2 and Example 3, i.e.1.7×10² and 2.2×10¹⁰.

EXAMPLE 5

In this example, both gases of PH₃ and NH₃ are used in the reactant gasmixture. The flow rate ratio of PH₃ to NH₃ is 0.75×10⁻² to 1. In thischemical vapor deposition process, the flow rate of silane is 1,000sccm, the flow rate of NH₃ is 200 sccm, the flow rate of PH₃ is 1.5sccm, the flow rate of H₂ is 1,000 sccm, the RF power used is 600 W, thepressure of the chamber is kept at 2.0 Torr, the spacing between theelectrodes is 962 mils (2.44 cm), the temperature of the susceptor iskept at 400° C., and a deposition rate of 197 nm/min is obtained. Thephysical properties of the films obtained are a tensile stress at1.3×10⁹ dyne/cm² and a resistivity at 9.6×10⁵ ohm-cm. It is seen that bydecreasing the flow rate of PH₃ relative to the flow rate of NH₃ theresistivity of the film when compared to Example 4 is increased and thetensile stress is only slightly increased.

EXAMPLE 6

In this example, both gases of NH₃ and PH₃ are used in the reactant gasmixture. The flow rate ratio of PH₃ to NH₃ is 1.5×10⁻² to 1. In thechemical vapor deposition process, a flow rate of silane at 1,000 sccmis used, a flow rate of PH₃ at 1.5 sccm is used, a flow rate of NH₃ at100 sccm is used, a flow rate of H₂ at 1,000 sccm is used, an RF powerof 600 W is used, a chamber pressure of 2.0 Torr is used, a spacingbetween the electrodes of 962 mils (2.44 cm) is used, a susceptortemperature of 400° C. is used, and a deposition rate of 240 nm/mm isobtained. Amorphous silicon films having properties of a tensile stressat 4.7×10⁹ dyne/cm² and a resistivity at 3.6×10⁵ ohm-cm are obtained. Itis seen in comparison to Example 5 that by decreasing the ammoniacontent in the reactant gas mixture as compared to that of PH₃, theresistivity of the deposited film is reduced and the tensile stress isslightly increased.

EXAMPLE 7

In this example, both gases of PH₃ and NH₃ are used in the reactant gasmixture. The flow rate ratio of PH₃ to NH₃ is 0.6×10⁻² to 1. It isbelieved that a flow rate ratio of PH₃:NH₃ as low as 0.1×10⁻² or 0.001:1may be used. In the chemical vapor deposition process, the flow rate ofsilane is 1,000 sccm, the flow rate of NH₃ is 200 sccm, the flow rate ofPH₃ is 1.5 sccm, the flow rate of H₂ is 1,000 sccm, the RF power used isreduced to 400 W, the pressure of the chamber is kept at 2.0 Torr, thespacing between the electrodes is 962 mils (2.44 cm), the temperature ofthe susceptor is kept at 400° C., and a deposition rate of 180 nm/min isobtained. The physical properties of the films obtained are a tensilestress at 6.3×10⁹ dyne/cm² and a resistivity at 7.0×10⁶ ohm-cm. It isseen that by increasing the flow rate of NH₃ as compared to Example 6,the resistivity of the film is greatly increased while the tensilestress almost remained constant.

EXAMPLE 8

In this example, both gases of PH₃ and NH₃ are used in the reactant gasmixture. The flow rate ratio of PH₃ to NH₃ is 2.5×10⁻² to 1. It isbelieved that a flow rate ratio of PH₃:NH₃ as high as 1×10-1 or 0.1:1may be used. In the chemical vapor deposition process, the flow rate ofsilane is 1,000 sccm, the flow rate of NH₃ is 100 sccm, the flow rate ofPH₃ is 2.5 sccm, the flow rate of H₂ is 1,000 sccm, the RF power used is400 W, the pressure of the chamber is kept at 2.0 Torr, the spacingbetween the electrodes is 962 mils (2.44 cm), the temperature of thesusceptor is kept at 400° C., and a deposition rate of 190 nm/min isobtained. The physical properties of the films obtained are a tensilestress at 4.8×10⁹ dyne/cm² and a resistivity at 6.0×10⁴ ohm-cm. It isseen that by increasing the flow rate of PH₃ and comparing to Example 7,the resistivity of the film is greatly decreased while the tensilestress remains essentially constant.

EXAMPLE 9

In Example 9, a doping gas of PH₃ and a gaseous nitrogen N₂ are used ina reactant gas mixture. The flow rate of silane is 1,000 sccm, the flowrate of PH₃ is 2.5 sccm, the flow rate of N₂ is 1,500 sccm, and the flowrate of H₂ is 1,000 sccm. The RF power used is 600 W, the pressure ofthe chamber is 1.2 torr, the spacing of the electrodes is 962 mils (2.44cm), the susceptor temperature is 400° C., and the deposition rateobtained is 190 nm/min. An amorphous silicon film having a desirableresistivity is obtained. The physical properties of the film aremeasured at a tensile stress of 2.5×10⁹ dyne/cm² and a resistivity of9.6×10⁵ ohm-cm. As expected, increasing the concentration of nitrogenincreases the resistivity and decreases the conductivity.

EXAMPLE 10

In Example 10, a doping gas of PH₃ and gaseous nitrogen N₂ are used inthe reactant gas mixture. The flow rate of silane is 1,000 sccm, theflow rate of PH₃ is 2.5 sccm, the flow rate of N, is 500 sccm, and theflow rate of H₂ is 1,500 sccm. The RF power used is 600 W, the pressureof the chamber is 1.2 torr, the spacing of the electrodes is 962 mils(2.44 cm), the susceptor temperature is 400° C., and the deposition rateobtained is 194 nm/min. An amorphous silicon film having a desirableresistivity is obtained. The physical properties of the film aremeasured at a compressive stress of 1.1×10⁹ dyne/cm² and a resistivityof 8.2×10² ohm-cm. The data is consistent with the results obtained onthe other examples.

Examples 1 through 3 shown above are comparative examples prepared byprior art methods. Examples 4 through 10 shown above illustrate theadvantages made possible by the present invention. The data for Examples1 through 10 are shown below in Tables 1 and 2.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 SIH4₄ 10001000 1000 1000 1000 sccm PH₃ — 5.5 — 2.5 1.5 sccm NH₃ — — 500 200 200sccm PH₃/PH₃ — — — 1.25 0.75 × 10² H₂ 1000 1000 1000 1000 1000 sccmpower 300 300 300 600 600 W pressure 2.0 2.0 2.0 2.0 2.0 torr spacing962 962 962 962 962 mil temp 410 410 410 400 400 ° C. dep rate 168 156135 197 197 nm/min stress C 8.0E8 C 1.7E9 T 7.4E9 T 4.0E8 T 1.3E9dyne/cm² resistivity 2.0E9 1.7E2 2.2E10 1.6E5 9.6E5 ohm-cm

TABLE 2 Example 6 Example 7 Example 8 Example 9 Example 10 SIH4₄ 10001000 1000 1000 1000 sccm PH₃ 1.5 1.5 2.5 2.5 2.5 sccm NH₃ 100 250 100 —— sccm PH₃/PH₃ 1.50 0.60 2.50 — — × 10² N₂ — — — 1500 500 sccm H₂ 10001000 1000 1000 1500 sccm power 600 400 400 600 600 W pressure 2.0 2.02.0 1.2 1.2 torr spacing 962 962 962 962 962 mil temp 400 400 400 400400 ° C. dep rate 240 180 190 190 194 nm/min stress T 4.7E9 T 6.3E9 T4.8E9 T 2.5E9 C 1.1E9 dyne/cm² resistivity 3.6E5 7.0E6 6.0E4 9.0E5 8.2E2ohm-cm

By mixing the doping gases of PH₃ and NH₃ together at various volumeratios, i.e. in the range between about 1000:1 and about 10:1 forNH₃:PH₃, amorphous silicon films having desirable stress values andresistivity values can be obtained. It is shown in FIG. 3 that bychanging the phosphine content in the reactant gas mixture, theconductivity of the films can be changed. For instance, increasing thephosphine content increases the conductivity of the film sincephosphorus is an electron donor. Similarly, changing the nitrogencontent in the reactant gas mixture changes the resistivity of the filmobtained since nitrogen contributes to the insulating property of thefilm. For instance, by increasing the nitrogen content of the reactantgas mixture, the resistivity of the amorphous silicon film obtained isincreased. A desirable range for the resistivity of the amorphoussilicon based films of the present invention is between about 10³ andabout 10⁷ ohm-cm. This range is obtainable as shown by the data shown inTable 1 and other experimental data not presented here. A preferredrange of resistivity is between about 10⁵ and about 10⁶ ohm-cm. Thus, byusing the inventive reactant gas mixture of PH₃ and NH₃, an amorphoussilicon-based film with a controlled conductivity and low stress may beformed.

The stress level in the films is generally preferred to be minimized.The stress values should be kept in the low 10⁹ dyne/cm² or in the high10⁸ dyne/cm² range. As seen in FIG. 4, the stress level remainsessentially constant with changing flow rate ratios of PH₃:NH₃. Thenovel method described herein enables a suitable control of the type ofstress in the amorphous silicon film deposited while enabling apredictable selection of the resulting resistivity.

Examples 1 through 8 used ammonia as the nitrogen-containing gas.Examples 9 and 10 used gaseous nitrogen N₂ as the nitrogen-containinggas and produced controllable resistivities in the range of 10² to 10⁴ohm-cm. It is believed that yet other nitrogen-containing gas such asN₂O would produce similar results. We believe that the nitrogen isintroduced into the amorphous silicon matrix at levels far above thoseassociated with doping levels. While increasing concentrations of thesemiconductor n-type dopant P from the PH₃ increases conductivities(decreases resistivities), increasing concentrations of nitrogen tend toincrease the electronic band gap as the resultant material progressivelychanges from amorphous silicon to silicon nitride. The larger band gapsare associated with increased resistivity. Thus, similar effects shouldbe obtainable by use of carbon-containing or oxygen-containing gaseswhich drive the material toward the semi-insulator silicon carbide andthe dielectric silicon dioxide.

For example, a carbon-containing volatile (e.g., a gas or a gaseoussubstance) may be introduced during the deposition of an amorphoussilicon-based film to control the conductivity of the deposited film. Inone embodiment, methane (CH₄) is introduced into the deposition chamberalong with a silicon-containing volatile (e.g., silane gas) and a dopantvolatile (e.g., PH₃ gas or B₂H₆ gas) to form an amorphous silicon-basedfilm. As shown in FIG. 5, when phosphine gas is used as the dopantvolatile, the resistivity of the deposited film increases as the flowrate of methane is increased relative to the flow rate of the phosphinegas (i.e., as the flow rate ratio PH₃:CH₄ decreases). Thus, the additionof a carbon-containing volatile into the deposition chamber tends toincrease the resistivity of the deposited film, assuming other factorsremain unchanged. In another embodiment, a nitrogen-containing volatile(e.g., NH₃, N₂, or N₂O) may be introduced into the deposition chamberalong with a carbon-containing volatile (e.g., CH₄) and a dopantvolatile (e.g., PH₃ or B₂H₆). In this embodiment, the carbon-containingvolatile and the nitrogen-containing volatile serve to increase theresistivity of the deposited film, and the dopant volatile serves toincrease the conductivity of the deposited film. In any of theseembodiments, the amorphous silicon-based film may be formed by many filmforming techniques, including chemical vapor deposition (CVD), plasmaenhanced chemical vapor deposition (PECVD), and physical vapordeposition (PVD) techniques. When the film is formed using PVD, thecarbon-containing volatile may be introduced into the deposition chamberby RF sputtering a high-purity silicon carbide target.

The examples described above show an amorphous silicon film having aresistivity controllable in the range of 10² to 10⁷ ohm-cm. We believethat the controllable range can be extended to 10¹⁰ ohm-cm by using verylimited amounts of phosphine, particularly with the heavy ammoniaconcentration of Example 3. The invention thus provides a method ofcontrollably achieving resistivities in the range of 10³ to 10⁹ ohm-cm,which were not easily obtainable in the prior art.

While the present invention has been described in an illustrativemanner, it should be understood that the terminology used is intended tobe in a nature of words of description rather than of limitation.

Furthermore, while the present invention has been described in terms ofa preferred embodiment thereof, it is to be appreciated that thoseskilled in the art will readily apply these teachings to other possiblevariations of the invention. For instance, other volume ratios betweenthe doping gases may be suitably used in place of those shown in theexamples. The dopant gas PH₃ of the examples provides n-type doping.Other n-type dopant gases may be used with the invention. Also, p-typedopant gases, such as B₂H₆, may be used with the invention. The hydrogengas of the examples is less reducing than NH₃ and thus acts primarily asa carrier gas although it is conventional to use hydrogen to deposithigh-quality amorphous silicon. Other carrier gases that are lessreducing than the nitrogen-containing gas can be substituted for the H₂.Examples of such carrier gases are Ar and He. Furthermore, even thoughthe process of PECVD is used to deposit layers in a field emissiondisplay device, other processes such as CVD may also be used to depositamorphous silicon based films that have controlled conductivity in othersemiconductor devices.

Other embodiments are within the scope of the claims.

What is claimed is:
 1. A method of forming an amorphous silicon-basedfilm on a substrate located inside a deposition chamber, the film havinga tensile stress of between about 10⁸ and about 10⁹ dyne/cm², the methodcomprising: introducing a silicon-based volatile into the depositionchamber; introducing into the deposition chamber aconductivity-increasing volatile including one or more components forincreasing the conductivity of the amorphous silicon-based film; andintroducing into the deposition chamber a conductivity-decreasingvolatile including one or more components for decreasing theconductivity of the amorphous silicon-based film; wherein theconductivity-increasing and conductivity-decreasing volatile areintroduced into said deposition chamber at a flow rate ratio betweenabout 1:1 and about 1:1000 conductivity-increasing toconductivity-decreasing volatile; thereby forming said amorphoussilicon-based film on said substrate.
 2. The method of claim 1, whereinthe flow rate ratio is selected to achieve a film resistivity of about10³-10⁷ ohm-cm.
 3. The method of claim 1, wherein theconductivity-increasing volatile consists of phosphine and theconductivity-decreasing volatile consists of ammonia, the phosphine andthe ammonia being introduced into the deposition chamber at a flow rateratio in a range of about 1:1000 to about 1:10 (phosphine:ammonia). 4.The method of claim 1, wherein the conductivity-increasing volatileconsists of phosphine and the conductivity-decreasing volatile consistsof methane, the phosphine and the methane being introduced into thedeposition chamber at a flow rate ratio in a range of about 1:100 toabout 1:1 (phosphine:methane).
 5. The method of claim 1, wherein theconductivity-increasing volatile includes a dopant.
 6. The method ofclaim 5, wherein the dopant includes an n-type dopant.
 7. The method ofclaim 6, wherein the n-type dopant includes phosphorous.
 8. The methodof claim 5, wherein the dopant includes a p-type dopant.
 9. The methodof claim 8, wherein the p-type dopant includes boron.
 10. The method ofclaim 1, wherein the amorphous silicon-based film is characterized by aband gap, and the conductivity-decreasing volatile includes a band gapincreasing component that increases the band gap of the amorphoussilicon-based film relative to a film formed under similar conditionsbut without the band gap increasing component.
 11. The method of claim1, wherein the conductivity-decreasing volatile includes nitrogen. 12.The method of claim 11, wherein the conductivity-decreasing volatileincludes ammonia, N₂, or N₂O.
 13. The method of claim 1, wherein theconductivity-decreasing volatile includes N₂O.
 14. The method of claim1, wherein the conductivity-decreasing volatile includes carbon.
 15. Themethod of claim 14, wherein the conductivity-decreasing volatileincludes methane.
 16. The method of claim 1, wherein the silicon-basedfilm consists of silane, the conductivity-increasing volatile consistsof phosphine, and the conductivity-decreasing volatile consists ofammonia.
 17. The method of claim 1, wherein the silicon-based filmconsists of silane, the conductivity-increasing volatile consists ofphosphine, and the conductivity-decreasing volatile consists of methane.18. The method of claim 1, further comprising introducing into thedeposition chamber a second conductivity-decreasing volatile.
 19. Themethod of claim 18, wherein the silicon-based film consists of silane,the conductivity-increasing volatile consists of phosphine, the firstconductivity-decreasing volatile consists of ammonia, and the secondconductivity-decreasing volatile consists of methane.
 20. The method ofclaim 1, wherein said amorphous silicon-based film has a filmresistivity of about 10⁵-10⁶ ohm-cm.
 21. The method of claim 1, whereinsaid amorphous silicon-based film has a film resistivity of about10²-10⁴ ohm-cm.
 22. The method of claim 1, wherein said amorphoussilicon-based film has a film resistivity of about 10⁷-10¹⁰ ohm-cm. 23.The method of claim 1, further comprising the step of introducing a H₂gas into the deposition chamber.
 24. The method of claim 23, whereinsaid introducing step comprises introducing said H₂ gas into saiddeposition chamber at a flow rate of about 1,000 to about 1,500 sccm.25. The method of claim 1, further comprising the step of heating saidsubstrate to a temperature between about 200° C. and about 400° C. 26.The method of claim 1, further comprising the step of heating saidsubstrate to a temperature between about 300° C. and about 350° C. 27.The method of claim 1 wherein said substrate is rectangular withapproximate dimensions of about 360 mm by about 465 mm.
 28. The methodof claim 1 wherein said amorphous silicon-based film is formed on saidsubstrate at a deposition rate of about 135 to about 194 nm/min.
 29. Amethod of forming an amorphous silicon-based film on a substrate,wherein said substrate is for a field emission display device, thesubstrate located inside a deposition chamber, the method comprising:introducing into the deposition chamber a silicon-based volatile;introducing into the deposition chamber a conductivity-increasingvolatile including one or more components for increasing theconductivity of the amorphous silicon-based film; and introducing intothe deposition chamber a conductivity-decreasing volatile including oneor more components for decreasing the conductivity of the amorphoussilicon-based film thereby forming said amorphous silicon-based film onsaid substrate.
 30. A method of forming an amorphous silicon-based filmon a substrate, wherein said film is a component of a flat panel displaydevice, the substrate located inside a deposition chamber, the methodcomprising: introducing into the deposition chamber a silicon-basedvolatile; introducing into the deposition chamber aconductivity-increasing volatile including one or more components forincreasing the conductivity of the amorphous silicon-based film; andintroducing into the deposition chamber a conductivity-decreasingvolatile including one or more components for decreasing theconductivity of the amorphous silicon-based film thereby forming saidamorphous silicon-based film on said substrate.
 31. A method of formingan amorphous silicon-based film on a substrate, wherein said film is acomponent of a field emission display device, the method comprising:introducing a silicon-based volatile into a deposition chamber;introducing a conductivity-increasing volatile into the depositionchamber at a first flow rate, the conductivity-increasing volatileincluding one or more components for increasing the conductivity of theamorphous silicon-based film; and introducing a conductivity-decreasingvolatile into the deposition chamber at a second flow rate, theconductivity-decreasing volatile including one or more components fordecreasing the conductivity of the amorphous silicon-based film; andregulating said first flow rate and said second flow rate so that theratio between said first flow rate and said second flow rate is betweenabout 1:1 and about 1:1000, thereby forming said amorphous silicon-basedfilm on said substrate.
 32. The method of claim 31, wherein saidamorphous silicon-based film has a film resistivity of about 10³-10⁷ohm-cm.
 33. The method of claim 31, wherein said amorphous silicon-basedfilm has a film resistivity of about 10⁵-10⁶ ohm-cm.
 34. The method ofclaim 31, wherein said amorphous silicon-based film has a filmresistivity of about 10²-10⁴ ohm-cm.
 35. The method of claim 31, whereinsaid amorphous silicon-based film has a film resistivity of about10⁷-10¹⁰ ohm-cm.
 36. The method of claim 31, wherein theconductivity-increasing volatile consists of phosphine and theconductivity-decreasing volatile consists of ammonia, and said flow rateratio is in a range of about 1:1000 to about 1:10 (phosphine:ammonia).37. The method of claim 31, wherein the conductivity-increasing volatileconsists of phosphine and the conductivity-decreasing volatile consistsof methane, and said flow rate ratio is in a range of about 1:100 toabout 1:1 (phosphine:methane).
 38. The method of claim 31, wherein theconductivity-increasing volatile includes a dopant.
 39. The method ofclaim 38, wherein the dopant includes an n-type dopant.
 40. The methodof claim 39, wherein the n-type dopant includes phosphorous.
 41. Themethod of claim 39, wherein the dopant includes a p-type dopant.
 42. Themethod of claim 41, wherein the p-type dopant includes boron.
 43. Themethod of claim 31, wherein the amorphous silicon-based film ischaracterized by a band gap, and the conductivity-decreasing volatileincludes a band gap increasing component that increases the band gap ofthe amorphous silicon-based film relative to a film formed under similarconditions but without the band gap increasing component.
 44. The methodof claim 31, wherein the conductivity-decreasing volatile includesnitrogen.
 45. The method of claim 44, wherein theconductivity-decreasing volatile includes ammonia, N₂, or N₂O.
 46. Themethod of claim 31, wherein the conductivity-decreasing volatileincludes carbon.
 47. The method of claim 46, wherein theconductivity-decreasing volatile includes methane.
 48. The method ofclaim 31, wherein the silicon-based film consists of silane, theconductivity-increasing volatile consists of phosphine, and theconductivity-decreasing volatile consists of ammonia.
 49. The method ofclaim 31, wherein the silicon-based film consists of silane, theconductivity-increasing volatile consists of phosphine, and theconductivity-decreasing volatile consists of methane.
 50. The method ofclaim 31, further comprising introducing into the deposition chamber asecond conductivity-decreasing volatile.
 51. The method of claim 50,wherein the silicon-based film consists of silane, theconductivity-increasing volatile consists of phosphine, the firstconductivity-decreasing volatile consists of ammonia, and the secondconductivity-decreasing volatile consists of methane.
 52. The method ofclaim 31, further comprising the step of introducing into H₂ gas intothe deposition chamber at a third flow rate.
 53. The method of claim 52,wherein the third flow rate is about 1,000 to about 1,500 sccm.
 54. Themethod of claim 31, further comprising the step of heating saidsubstrate to a temperature between about 200° C. and about 400° C. 55.The method of claim 31, further comprising the step of heating saidsubstrate to a temperature between about 300° C. and about 350° C. 56.The method of claim 31, wherein a stress level of said amorphoussilicon-based film on said substrate is between about 10⁸ dyne/cm² and10⁹ dyne/cm².
 57. A method of forming an amorphous silicon-based film ona substrate, the method comprising: maintaining a silicon-based volatileat a first partial pressure; maintaining a conductivity-increasingvolatile at a second partial pressure, the conductivity-increasingvolatile including one or more components for increasing theconductivity of the amorphous silicon-based film; and maintaining aconductivity-decreasing volatile at a third partial pressure, theconductivity-decreasing volatile including one or more components fordecreasing the conductivity of the amorphous silicon-based film; andregulating said first, second and third partial pressures to therebyform said amorphous silicon-based film on said substrate such that saidamorphous silicon-based film has a stress level of about 10⁸ dyne/cm² toabout 10⁹ dyne/cm².
 58. The method of claim 57, the method furthercomprising maintaining a total partial pressure in said depositionchamber between about 0.5 torr and about 5 torr.
 59. The method of claim58, the method further comprising maintaining a total partial pressurein said deposition chamber between about 1.5 torr and about 2.5 torr.60. A method of forming an amorphous silicon-based film on a substrate,wherein said film is a component of a flat panel display device, themethod comprising: maintaining a silicon-based volatile at a firstpartial pressure; maintaining a conductivity-increasing volatile at asecond partial pressure, the conductivity-increasing volatile includingone or more components for increasing the conductivity of the amorphoussilicon-based film; and maintaining a conductivity-decreasing volatileat a third partial pressure, the conductivity-decreasing volatileincluding one or more components for decreasing the conductivity of theamorphous silicon-based film; and regulating said first, second andthird partial pressures to thereby form said amorphous silicon-basedfilm on said substrate such that said amorphous silicon-based film has aresistivity of about 10³ ohm-cm to about 10⁷ ohm-cm.
 61. A method offorming an amorphous silicon-based film on a substrate located inside adeposition chamber by a plasma-enhanced chemical vapor depositionprocess, the method comprising: introducing into the deposition chambera silicon-based volatile; introducing into the deposition chamber aconductivity-increasing volatile including one or more components forincreasing the conductivity of the amorphous silicon-based film; andintroducing into the deposition chamber a conductivity-decreasingvolatile including one or more components for decreasing theconductivity of the amorphous silicon-based film; wherein: saidplasma-enhanced chemical vapor deposition process is limited to a plasmapower of about 0.18 watts/cm² to about 0.36 watts/cm².